Add liquid.cr support (#15)

* Add liquid.cr support

* Minor

* Add specs for passing custom context to Liquid

E.g.:

context = Liquid::Context.new
context.set "key", "value"
render( "/path/to/template.liquid", context)

spec/fixtures/test.liquid

@@ -0,0 +1 @@
+<span>{{ process.pid }}</span>

spec/kilt/liquid_spec.cr

@@ -0,0 +1,31 @@
+require "../spec_helper"
+require "../../src/liquid"
+
+class LiquidView
+  @process = { "pid" => Process.pid }
+  Kilt.file "spec/fixtures/test.liquid"
+end
+
+class LiquidViewWithCustomContext
+  # Use of instance variable is not required in user code. It is used here to
+  # avoid name clash with 'context' variable existing within spec.
+  def initialize
+    @context = Liquid::Context.new
+    @context.set "process", { "pid" => Process.pid }
+  end
+  Kilt.file "spec/fixtures/test.liquid", "__kilt_io__", "@context"
+end
+
+it "renders liquid" do
+  ctx = Liquid::Context.new
+  ctx.set "process", { "pid" => Process.pid }
+  Kilt.render("spec/fixtures/test.liquid", ctx).should eq("<span>#{Process.pid}</span>\n")
+end
+
+it "works with classes" do
+  LiquidView.new.to_s.should eq("<span>#{Process.pid}</span>\n")
+end
+
+it "works with classes and custom context" do
+  LiquidViewWithCustomContext.new.to_s.should eq("<span>#{Process.pid}</span>\n")
+end